Issue with exporting .STL after shade smoothing
Shade smooth is only shading done in software and doesn't change the geometry of your model.  If you want it smoother you need to add a subdivision surface modifier which will add the &quot;smoothness&quot; to your geometry.
As you have imported this as an STL, the topology will be pretty bad and the subdivision surface modifier may not work well (which may be why meshmixer is giving you trouble too). Ideally, you want to remesh it first.  Decimate might work depending on the model and then add subdivision surface modifier afterwards to smooth it.
Remember when modelling for 3d printing, model with shade flat, not shade smooth.